In hypertension, renal vascular resistance increases, shifting the pressure natriuresis curve to the right. The afferent arteriole (Af-Art) accounts for most renal vascular resistance. It controls glomerular filtration rate and renal function. Af-Art resistance is regulated by factors similar to those that control other arterioles. In addition, the Af-Art is controlled by tubuloglomerular feedback (TGF). TGF operates via the macula densa, which senses increases in luminal NaCI and sends a signal to constrict the Af-Art. The kidney expresses heme oxygenases (HOs), which release carbon monoxide (CO), biliverdin, and ferrous iron. We have evidence that HO inhibition potentiates TGF, while CO and biliverdin reduce it. We hypothesize that HOs in the macula densa produce CO and biliverdin, both of which act synergistically and in an autocrine manner to inhibit TGF. CO acts by inhibiting the Na-K-2CI cotransporter (NKCC2) and by blocking ATP release. Biliverdin acts by decreasing superoxide (O2-), thereby increasing NO bioavailability. This hypothesis will be tested in 4 aims: Aim I. Hypothesis: HO-2 in the macula densa releases CO, which inhibits TGF in an autocrine manner. Aim II. Hypothesis: CO inhibits TGF by stimulating cGMP production. cGMP inhibits TGF in part by activating cGMP-dependent protein kinase and cGMP-stimulated phosphodiesterase 2, thereby reducing cAMP concentration. A decrease in cAMP results in decreased NKCC2-dependent Na entry. Aim III. Hypothesis: CO-induced cGMP production inhibits TGF, in part, by blocking depolarization-induced Ca entry and/or inhibiting maxi-anion channel activity and ATP release. Aim IV. Hypothesis: HO-2 in the macula densa releases biliverdin, which decreases O2-. The decrease in O2- increases NO bioavailability. Via these mechanisms biliverdin inhibits TGF and potentiates the effects of CO. We will study TGF in vitro and in vivo. In addition to pharmacological probes, mice with HO-2, -1, or nitric oxide synthase 1gene deletion will be used. This project relates to the central theme because we will study autocrine factors (CO and biliverdin) produced by the macula densa cells that participate in the regulation of renal microcirculation, and as a result play a key role in regulating renal function. The information from this project will be integrated with that from all other projects. It will use all of the cores. The hypothesis that CO regulates TGF is a new paradigm and will lead to a better understanding of the mechanisms that control renal microcirculation and function.